Potentiometric sensors usually measure potential difference between a working electrode and a reference electrode, with the working electrode being arranged in a buffer solution, which is separated by a glass membrane from the medium to be investigated. The reference electrode is placed in a reference electrolyte; in the case of a pH-sensor, the reference electrolyte can be a 3.5 molar KCl-solution.
For assuring constant quality of measurement, the reference electrolyte must be renewed at appropriate times. To this end, pH- or redox-sensors usually have a replenishment opening on their exterior surface. Replenishment of the reference electrolyte proves to be cumbersome, in so far as operating personnel must go to each sensor individually, in order to perform replenishment through its replenishment opening. For facilitating this procedure, pH-sensors are available, which have a tube connection, via which reference electrolyte can flow-in from a reservoir. The tube connection can be, for example, a tube-nipple of glass on the replenishment opening, or it can be integrated into a sensor head. Similarly, the Jumo company offers a so-called pressure head, which has a tube connection for the reference electrolyte.
All of the described sensors are, in accordance with their use, screwed into a process opening. As this happens, a sealing ring, as process seal, is compressed between sealing surfaces on the sensor and on the process opening. Associated with this prior art is, however, a safety risk in respect that an extra rotation of the tube connection with respect to the process opening, for instance for bringing the tube connection into the right orientation, can lead to an over-tightening of the process seal.
Additionally, the pressure head of the Jumo company has the following safety deficiency. A sealing ring, which seals the path of the reference liquid feed against the shaft of the sensor externally of the path, is pressed by means of a screw-in sleeve into its sealing seat and against the sensor shaft. The screw-in sleeve is threaded at both ends. A first thread serves for screwing the screw-in sleeve into the sensor head, for compressing the sealing ring in the described manner. The second thread serves as the process connection thread, for screwing the sensor into a process connection. A turning of the sensor head thus endangers, in this case, two seals, namely the process seal and the seal of the reference liquid feed. In an extreme case wherein both seals would fail, it is then possible for reference liquid to get into the process medium which is being measured.